SIE G. STOKESj BART., ON THE PERCEPTION OF COLOUR. 265 



in all three cases, but the part of the body to which the 

 sensation is referred is in each case the same, whether tlie 

 sensation be that of pressure or warmth, or as it may be a 

 mixture of the two ; and tliat, although in accordance with 

 experiments already referred to, it appears tJiat it is by two 

 different sets of nerve fibres that the sensations of pressure 

 and of warmth respectively are conveyed. While the 

 character of the sensation (be it of pressure or heat) 

 depends very probably on the part of the brain to which 

 the nerves of the three sets lead, the part of the body to 

 which the sensation is referred seems to depend on the 

 position of the distal ends of the nerves. 



It would be in full accordance Avith this to suppose that 

 when the nerve fibres belonging to a particular cone of the 

 retina are stimulated by the rays from a luminous point 

 which are there brought to a focus, while the character of 

 the sensation as to whiteness or colour depends on the pro- 

 portion in which the three supposed sets of nerve fibres 

 are stimulated, which itself depends on the character of the 

 light, the part of the body to which the sensation is referred is 

 the particular cone in question, the same therefore for all three 

 of the primary colour sensations. Different luminous points 

 are seen in the same order of sequence in which their images 

 lie in the retina. Furthermore, just as in touching in the 

 dark an object with the forefinger we can judge of the 

 position of the object relatively to our body, of whether it 

 lies right or left, up or down, through the knowledge we 

 have of the position of the arm, so in vision we can not only 

 judge by direct sensation of the position of an object 

 relatively to the point we are directly looking at, but also 

 as to the direction of such an object relatively to a point 

 right opposite to the head, through the knowledge we have 

 of the way in which we have willed to turn the eye-balls 

 when the object is in our field of vision. 



The contrast between our perceptions of sound and light 

 may be emphasised by saying that while both phenomena 

 objectively considered depend on undulations, in sound we 

 have a direct perception of frequency, but not of direction, 

 while in light we have a direct perception of direction, 

 but not of frequency. This succinct statement requires 

 explanation, without Avhich it might even be supposed to 

 be untrue. It might be said, we have a continuous change 

 of pitch, from the lowest bass to the shrillest sound that we 

 can hear, and we have a continuous change of hue from the 



